Consumer demand for additional storage capacity on the optical storage medium is high. Various manufacturers are vying to develop new technologies for that purpose.
A technique to increase the storage capacity of an optical storage medium is to increase the storage density of a storage layer in the optical storage medium. An increased storage density for a storage layer is achieved by reducing the size of recording marks written in the storage layer to store information on the optical storage medium. The recording mark is written by shining a beam of light onto the storage layer. To write a small recording mark, the beam needs to be focused to form a spot with a small diameter on the storage layer. The diameter of the spot on the storage layer is in direct proportion to the numerical aperture (NA) of an objective lens focusing the beam and in inverse proportion to the wavelength of the beam. These properties have been exploited to large capacity optical storage media with high storage density by increasing the numerical aperture of the objective lens in the optical pickup and reducing the wavelength of a beam emitted by a light source in the optical pickup.
Another technique is to accommodate a plurality of storage layers in the optical storage medium. For example, an optical storage medium with two storage layers has in principle double the storage capacity of a medium with a single storage layer. Taking read/write margins into consideration, commercialized optical storage media with two storage layers have 1.5 times to twice the storage capacity of an optical storage medium with one storage layer.
Table 1 shows several examples of large capacity optical storage media available on the market. As could be appreciated from Table 1, commercial products vary greatly: some of them deliver increased capacity by increasing storage density, whilst others do so by containing dual storage layers.
TABLE 1CD-ROMDVD-ROMDVD-RDVD-RWNo. of Layers112121Storage Density (GB)0.74.78.54.78.54.7λ of Read Laser (nm)780650650/635Thickness of1.20.6Protection Layer (mm)NA of Objective Lens0.450.6
Several types of optical storage media has become commercially available as listed above, in a general trend toward large capacity optical storage media. Meanwhile, every optical read/write device for recording/reproducing information to/from an optical storage medium can handle limited types of media. The situation has inevitably given rise to compatibility problems. A particularly serious problem is that when a new type of optical storage medium is introduced to the market, existent optical read/write devices is not capable of recording/reproducing information to/from the optical storage medium.
A solution to the compatibility problems between optical storage media and optical read/write devices is so-called “multi-format optical storage media” which contain different types of storage layers in a single optical medium body. Table 2 shows an example of the multi-format optical storage media available on the market.
TABLE 2Super Audio CD (Hybrid)CD LayerDVD LayerStorage Density (GB)0.74.7λ of Read Laser (nm)780650Thickness of1.20.6Protection Layer (mm)NA of Objective Lens0.450.6
The multi-format optical storage medium in Table 2 contains two storage layers, one for DVD format and another for CD format. The medium is a so-called “single-sided read-out” type of multi-format optical storage medium: information is read only from one side by shining light onto the medium in such a manner that information is read from the two storage layers independently. The provision of both the DVD-format and CD-format storage layers enables the multi-format optical storage medium to record a song in two different levels of sound quality. If a user who owns a CD player (no DVD playback) buys such a multi-format optical storage medium, he can play the medium for high quality DVD music when he later buys an optical read/write device with a DVD playback capability. A user who owns both a CD player (no DVD-playback) and a DVD player (no CD-playback) can play the medium for the same music, albeit with different sound quality, on both of the devices.
Amid this ongoing trend of commercialization of a great variety of optical storage media, there is a strong demand for the advent of the multi-format optical storage medium which are compatible with all these formats. For example, a multi-format optical storage medium with a single HD-DVD layer and a single DVD layer is reported in non-patent document 1 entitled “Development of single-sided double layer disc for HD DVD and DVD playback,” dated Dec. 7, 2004, Toshiba Corporation, available on the WWW at <http://www.toshiba.co.jp/about/press/2004—12/pr_j0701.htm> (URL last checked on May 18, 2005).
In this particular multi-format optical storage medium, light must travel different distances through a protection layer or layers before it is focused onto different storage layers. It is difficult to use a common objective lens in an optical pickup to write or read recording marks in different storage layers in the multi-format optical storage medium. The trouble lies with difficulty in using a single objective lens and at the same time controlling spherical aberration within a tolerance for a plurality of storage layers.
Now, spherical aberration problems will be described in detail.
An optical pickup in an optical read/write device has an objective lens which focuses a beam of light emitted by a light source in the optical pickup onto a storage layer in an optical storage medium.
The objective lens is designed assuming a protection layer with a certain thickness. The magnitude of spherical aberration at the spot formed by the focused beam after the beam has passed through the protection layer having that thickness is regarded as a measure for good design. In other words, the objective lens is designed to focus a beam of light so that the beam can pass through a protection layer with an assumed thickness and form a focused spot with minimum spherical aberration. By “designing an objective lens,” we mean selection of materials for the objective lens and determination of the shapes of lens surfaces, and the distance between the lens surfaces, and other factors.
Put differently, spherical aberration occurs on the storage layer if the beam focused by the objective lens passes through a protection layer which has a different thickness from the thickness for which the objective lens is designed to minimize the spherical aberration, before forming a spot on the storage layer. The spherical aberration on the storage layer increases with an increase in the difference between the thickness of the protection layer which was assumed in the designing of the objective lens to minimize the spherical aberration and the thickness of the protection layer which the beam focused by the objective lens actually travels before forming a spot on the storage layer. The difference will be hereinafter referred to as the protection layer thickness error.
Large spherical aberration results in insufficient beam intensity at the focused spot. In reading the storage layer, spherical aberration beyond a tolerance level on the storage layer undesirably reduces the amplitude of a read signal. In addition, recording marks are difficult to form in writing the storage layer.
Therefore, the optical pickup needs to be equipped with an objective lens fine-tuned to match the thickness of the protection layer in the optical storage medium so that the spherical aberration which occurs on the storage layer to be read/written by the optical pickup remains within a tolerance.
Any multi-format optical storage medium contains a plurality of storage layers coexisting in a single optical medium body. The thickness of the protection layer(s) traveled by the beam emitted from an optical pickup before forming a spot on a storage layer differs from one layer to the other. Therefore, no matter any thickness is assumed for the protection layer(s) in the design process of the objective lens, thickness error occurs inevitably for a protection layer(s) having a different thickness from the thickness for which the objective lens is designed to minimize the spherical aberration. That thickness error in turn causes spherical aberration on the storage layer associated with the protection layer exhibiting the thickness error.
As the multi-format optical storage medium accommodates more storage layers, the protection layers in the multi-format optical storage medium increasingly differ in thickness from each other, and it becomes increasingly difficult to keep the spherical aberration within a tolerance for all the storage layers in the multi-format optical storage medium.
Furthermore, for the same thickness error, the spherical aberration grows with a decrease in the wavelength of the beam with which recording marks are written or read in the storage layer in the multi-format optical storage medium. In other words, it becomes even more difficult to keep the spherical aberration within a tolerance on a multi-format optical storage medium having a high density storage layer which requires use of a short wavelength to write small recording marks.
These spherical aberration problems can be addressed by providing the optical pickup with a plurality of objective lenses which correspond to the plurality of storage layers in the multi-format optical storage medium. However, the provision of the plurality of objective lenses in the optical pickup leads to a new set of problems: the structure of the optical pickup becomes complicated, and the manufacturing cost of the optical pickup increases.